Role of Water During Crystallization of Amorphous Cobalt Phosphate Nanoparticles

The transformation of amorphous precursors into crystalline solids and the associated mechanisms are still poorly understood. We illuminate the formation and reactivity of an amorphous cobalt phosphate hydrate precursor and the role of water for its crystallization process. Amorphous cobalt phosphate hydrate nanoparticles (ACP) with diameters of ∼20 nm were prepared in the absence of additives from aqueous solutions at low concentrations and with short reaction times. To avoid the kinetically controlled transformation of metastable ACP into crystalline Co3(PO4)2 × 8 H2O (CPO) its separation must be fast. The crystallinity of ACP could be controlled through the temperature during precipitation. A second amorphous phase (HT-ACP) containing less water and anhydrous Co3(PO4)2 was formed at higher temperature by the release of coordinating water. ACP contains approximately five molecules of structural water per formula unit as determined by thermal analysis (TGA) and quantitative IR spectroscopy. The Co2+ coor...